1. Field of the Invention
The present invention relates to a secondary battery and, more particularly, to a cap assembly of a secondary battery, which is designed to seal the battery housing more effectively and thus prevent electrolyte from leaking.
2. Description of Related Art
Generally, secondary batteries are made compact in size and rechargeable, and yet have a large capacity. Well known as secondary batteries are a Ni--MH battery, a Li battery, and a Li-ion battery.
Particularly, the Li-ion battery uses lithium metal oxide as a positive active material and carbon or a carbon compound as a negative active material. An organic solvent in which lithium salt is dissolved is used as the electrolyte via which the lithium ions move between positive and negative electrodes. Batteries are charged and discharged as the lithium ions are moved between positive and negative electrodes.
FIG. 4 shows a conventional Li-ion battery.
A rolled electrode assembly 2 is inserted into a can 4 filled with electrolyte, the rolled electrode assembly 2 comprising of positive and negative electrodes and a separator disposed between the positive and negative electrodes. Mounted on an upper end of the can 4 is a cap assembly 6.
The cap assembly 6 is insulated from the can 4 by a gasket 8. That is, the upper end of the can 4 is crimped on an outer periphery of the cap assembly 6 with the gasket 8 in between. The cap assembly 6 comprises a plate 14 provided with a safety groove, a current control member 12 disposed on the plate 14, and a cap cover 10 disposed on the current control member 12. Attached under the plate 14 is a circuit breaker 18 with an insulator 16 disposed therebetween. The insulator 16 is supported by a support plate 17, which is designed to maintain its attaching state even when the circuit breaker 18 is detached from the plate 14.
The circuit breaker 18 is welded on a positive lead 20 so that it is electrically coupled to a positive electrode of the electrode assembly 2.
In the conventional Li-ion battery structured as described above, to enhance airtightness and prevent the electrolyte from leaking, the gasket 8 is designed to be compressed by a predetermined level of pressure. However, if the gasket 8 is mis-assembled, the degree of airtightness may degrade, thereby allowing the electrolyte to leak.
Particularly, in the case of a cylindrical Li-ion battery, airtightness depends on the contacting area between the plate 14 and the gasket 8. However, in the conventional battery, the contacting area is insufficient and the housing may be not airtight.